Characteristics of fracture propagation and permeability response of sandstone under cyclic impact effect

To investigate the fracture and permeability characteristics of sandstone-type uranium ore under cyclic impact, a Hopkinson bar experimental system was used to load sandstone samples by cyclic impacts. The dynamic mechanical properties of the sandstone samples were measured after 3, 6 and 9 impacts. Subsequently, the impacted sandstone samples were subjected to CT scanning, and the crack images obtained from the scans were reconstructed in three-dimensions to measure the changes in pore and fracture parameters. The internal structures and damages in the impacted samples were then analyzed. Furthermore, a microscopic seepage simulation was performed to analyze the permeability of the samples, revealing the changes in the simulated permeability. Finally, permeability tests were conducted on the impacted samples to measure the variations in the actual permeability. Results show that cyclic impacts cause cumulative damage in the specimens, reducing their dynamic mechanical properties. As the number of impacts increases, energy in the specimens accumulates and releases cyclically. This cyclic accumulation and release of energy lead to a process of crack "expansion, compaction, re-expansion, re-compaction". During the cyclic impact process, small and isolated cracks inside the specimen gradually develop into larger, interconnected fractures. Simultaneously, medium-sized cracks exhibit dual effects of faulting and connectivity, presenting nonlinear characteristics. Cyclic impacts induce more complex fractures in the specimens, leading to an increased number of fluid seepage pathways and a larger scale of seepage. When subjected to three cycles of impact, the sample forms a single crack, resulting in a permeability increase of 340.91%−380.00%. After six cycles of impact, the cracks begin to connect, leading to a permeability increase of 1468.18%−2893.33%. With nine cycles of impact, a connected network of cracks forms, resulting in a permeability increase of 4718.18%−9380.00%. The cyclic impact significantly enhances the permeability of sandstone, with crack propagation and connectivity being the key driving factors for the increase in permeability.